1. Field of the Invention
This invention relates to a method for forming a ferroelectric domain-inverted structure, more particularly a method for forming a ferroelectric domain-inverted structure which is preferably usable for forming a periodic ferroelectric domain-inverted structure in an optical waveguide device suitable for a second harmonic-generation (SHG) device utilizing a Quasi-Phase-Matching (QPM) system.
2. Related Art Statement
As a light source for a blue laser usable for an optical pickup, etc., it is desirable to employ a SHG device utilizing a QPM system, comprising an optical waveguide made of lithium niobate single crystal or lithium tantalate single crystal which has a periodic ferroelectric domain-inverted structure. The device may be widely used in the fields of optical disk memory, medicine, and optochemistry, and can also be used as an optical pickup for various optical measurement devices.
The periodic ferroelectric domain-inverted structure may be formed on a ferroelectric single crystal such as lithium niobate by a titanium indiffusion method, a lithium oxide out-diffusion method, a silicon dioxide cladding and heat treatment method, a proton-exchanging and heat treatment method, an electron beam-scanning method, an electric voltage applying method, or the like. Because it is capable of forming a uniform and deep ferroelectric domain-inverted layer, the electric voltage applying method is most effectively used.
Moreover, the technique of ferroelectric domain-inverting a ferroelectric single substrate such as lithium niobate by the electric voltage applying method is effective in the case of forming a periodic ferroelectric domain-inverted structure for a QPM-SHG device.
For example, in xe2x80x9cJournal of Lightwave Technologyxe2x80x9d, Vol.14, No.3, March, 1996, p462xcx9c468, the condition for ferroelectric domain-inverting a substrate made of lithium niobate single crystal by the electric voltage applying method is described in detail. Furthermore, in xe2x80x9cElectronics Lett.xe2x80x9d, Vol.32, No.22, Oct. 24, 1996, p2091xcx9c2092, an example of domain-inverting a substrate made of MgO doped lithium niobate single crystal by the electric voltage applying method is described. In xe2x80x9cAppl. Phys. Lett.xe2x80x9d, 70(10), March, 1997, p1201xcx9c1203, an example of ferroelectric domain-inverting a substrate made of lithium tantalate single crystal is explained.
However, a substrate made of a bulk single crystal except the above lithium niobate, MgO doped lithium niobate single crystal and lithium tantalate single crystal has hardly been investigated.
The inventors have investigated a method that, as shown in FIG. 1, a film 2 made of ferroelectric single crystal is formed on an X-cut face of a substrate 1 made of ferroelectric single crystal by a liquid-phase epitaxial method and a comb-shaped electrode 3 and a uniform electrode 4 are formed on the ferroelectric single crystalline film 2, and thereafter the film 2 is polarized in the Z-direction.
However, when the same condition of applying an electric voltage as that in the above conventional techniques is employed in the above method as shown in FIG. 1, the film cannot be ferroelectric domain-inverted.
It is an object of the present invention to provide a method for easily forming ferroelectric domain-inverted structures, containing finely inverted domains, in a material that is otherwise resistant to the formation of such structures.
It is another object of the present invention to provide an easy method for ferroelectric domain-inverting, by the electric voltage applying method, a ferroelectric single crystal whose ferroelectric domain-inverting conditions are not necessarily clear.
It is still another object of the present invention to provide a method for changing a ferroelectric domain-inverted area when a ferroelectric domain-inverted structure is formed by the electric voltage applying method.
This invention relates to a method for forming a ferroelectric domain-inverted structure, comprising the steps of:
joining at least two kinds of ferroelectric material which have different spontaneous polarizations, and
ferroelectric domain-inverting one ferroelectric material among the ferroelectric materials and thereby ferroelectric domain-inverting the other ferroelectric material joined thereto.
A ferroelectric domain-inverted structure in a ferroelectric memory or a SHG device is required to have finely inversed domains with a dimension of sub-microns to several microns. Forming a ferroelectric domain-inverted structure having finely inverted domains is difficult because of the following two reasons:
1. Forming fine ferroelectric domain-inverted nuclei is sometimes difficult depending on the material, and
2. Finely ferroelectric domain-inverted structures tend to collapse as ferroelectric domain-inverted parts grow from the ferroelectric domain-inverted nuclei.
The ferroelectric domain-inversion starts when the ferroelectric domain-inverted nuclei generate and the ferroelectric domain-inverted structure is formed. When the characteristics of the applied electric voltage and the ferroelectric domain-inversion are investigated in detail, the applied electric voltage is more slightly increased to an inversion-voltage at the beginning of the ferroelectric domain-inverted nuclei""s generation.
Depending upon the material, however, the ferroelectric domain-inverted nuclei are very unlikely to generate, or the ferroelectric domain-inversion occurs in a whole range without the generation of the ferroelectric domain-inverted nuclei. For example, since the generating voltage of the ferroelectric domain-inverted nuclei is very different from the inversion-voltage in a material that is resistant to generation of the ferroelectric domain-inverted nuclei, the ferroelectric domain-inversion is rapidly progressed just after the generation of the ferroelectric domain-inverted nuclei. Thus, it is very difficult to control the configuration of the ferroelectric domain-inverted parts.
On the other hand, in a material without the generation of the ferroelectric domain-inverted nuclei, voltage-applied parts thereof are easily inverted. In this case, the ferroelectric domain-inversion occurs in the whole of the parts around an electrode, so that the ferroelectric domain-inversion""s configuration can not be controlled by a fine electrode structure.
The present invention is based on the discovery of the above fact.